Consideration as to producing sufficiently homogeneous, hardenable low alloy powdered steel for processing as preforms for hot forming or as sintered shapes involves either or both of two procedures: pre-alloying or admixing. 100% pre-alloyed powders are currently in use as the basic material for low-alloy steel preforms or compacted shapes because of their homogeneity. However, 100% pre-alloyed powders are relatively expensive compared to iron powder or conventionally produced iron and it is unlikely that parts producers will accept the limited number of alloyed compositions commercially available. Accordingly, 100% pre-alloyed powders properly represent only one of several means of providing a full range of alloy preforms which are substitutional for conventionally made wrought alloy compositions.
Mechanical mixtures of iron and alloy powders, hereinafter referred to as admixtures, have been deemed capable of providing alloying during sintering of the precompact, but exactly how to achieve adequate homogenization of the alloying ingredients is not known to the prior art. The prior art recognizes that conceptually, admixtures seem to offer substantial economic advantages over 100% pre-alloyed powders. Complete flexibility should result from blending a base iron powder with a master alloy powder and thereby achieve great reduction in manufacturing costs (fuel and alloying ingredients). To arrive at this goal, there must be a different preparation of the master alloy powder and the total admixture must be designed to improve the kinetics of the sintering process.
A variety of mechanisms are at hand to produce the alloying condition by diffusion with degrees of success. For example, solid state particle diffusion can be used, diffusion resulting from gasification of one of the components to the admixture is feasible, or liquid phase sintering of the master alloy portion can be employed. The prior art (i.e. U.S. Pat. No. 2,489,838) demands solid or gas state diffusion in many cases because it was believed liquid phase diffusion caused excessive shrinkage and distortion. But this approach ignores the efficiency of alloying. Since diffusion in the solid state particle condition is limited by the number of the inner particle contacts, the hope of increasing the kinetics of complete alloying is limited. If the master alloy ingredient is converted to a gas or a liquid, there is an increase in the inner particle contact. But very few elements can be considered for the technique of gasification to one of the compounds and thus this avenue is relatively narrow in application. Therefore, there is a need for exploration and development of a method by which a master alloy powder will function by liquid phase sintering.
The use of an iron-carbon eutectic as a base for a master alloy to behave much as copper in a standard production alloy during sintering was known more than 20 years ago. (See U.S. Pat. No. 2,238,382 to Boegehold). Unlike nonferrous alloying additions, these master alloys were found to have much greater solubility. However, certain problems must be overcome if the advantageous solubility of iron-iron carbide eutectic is to be commercially utilized. The carbon in the eutectic powder diffuses out during heating before the eutectic is fully liquified thus raising its melting temperature and resulting in islands of unmelted alloy. The carbon is fixed in ratio within the eutectic powder and limits the design of different hardenability responses.
The liquid phase process must work with only two steps of compacting and heating. The ingredients of a master alloy powder must be selected with care so that each of the ingredients is compatible one with the other to liquify together in a melting range which is relatively narrow and as low as possible; the master alloy powder must have good fluidity and wetting characteristics to facilitate coating of the base ferrous powder with the alloy liquid for purposes of facilitating rapid and effective sintering and diffusion through a minimum distance. The ingredients of the master alloy must not contain deleterious amounts of elements, such as excessive silicon (see U.S. Pat. No. 3,689,257) which produces poor physical characteristics in the final product.